A water absorbent resin (Super Absorbent Polymer; abbreviated as SAP) has been used in a wide range of uses for sanitary materials such as paper diapers, sanitary napkins, incontinence products for adults, and the like, and uses for water retention agent for soil, owing to properties that the resin can absorb a large quantity of a water-based liquid several times to several hundred times as much as the mass of itself and has been manufactured and consumed in large quantities.
In general, a water absorbent resin is produced by polymerizing an aqueous solution containing a hydrophilic monomer and a crosslinking agent to obtain a hydrous gel-like polymer, drying the gel polymer, and surface-crosslinking the dried product. The physical properties such as water absorption against pressure (AAP) and liquid permeability (GBP, SFC) of the above-mentioned water absorbent resin are improved by surface-crosslinking step. The surface-crosslinking step is commonly a step of providing a highly crosslinked layer in the vicinity of the water absorbent resin surface by causing reaction of the water absorbent resin with a surface-crosslinking agent or a polymerizable monomer.
Various kinds of surface-crosslinking agents reactive on a functional group of a water absorbent resin (particularly, carboxyl group) are proposed as a surface-reforming method of such a water absorbent resin and examples known as the surface-crosslinking agents are oxazoline compounds (Patent Document 1), vinyl ether compounds (Patent Document 2), epoxy compounds (Patent Document 3), oxetane compounds (Patent Document 4), polyhydric alcohol compounds (Patent Document 5), polyamide polyamine-epihalo adducts (Patent Documents 6, 7), hydroxyacrylamide compounds (Patent Document 8), oxazolidinone compounds (Patent Documents 9, 10), bis- or poly-oxazoline compounds (Patent Document 11), 2-oxotetrahydro-1,3-oxazolidine compounds (Patent Document 12), alkylene carbonate compounds (Patent Document 13), and the like. A technique using a specified surface-crosslinking agent (Patent Document 14) is also known.
Techniques also known as the surface-reforming method other than the method carried out by a surface-crosslinking agent are a technique of surface-crosslinking by polymerizing a monomer in the vicinity of the water absorbent resin surface (Patent Document 15) and techniques of radical crosslinking with persulfuric acid salts etc. (Patent Documents 16, 17). Techniques of reforming water absorbent resins by heating without using a surface-crosslinking agent (Patent Documents 18, 19), which is different from common surface-crosslinking treatment, are also known.
A technique of using an additive in combination for mixing a surface-crosslinking agent is also proposed and examples known as the additive are water-soluble cations such as aluminum salts and the like (Patent Documents 20, 21), alkali (Patent Document 22), organic acids or inorganic acids (Patent Document 23), peroxides (Patent Document 24), and surfactants (Patent Document 25).
Not only the chemical methods but also many surface treatment methods using apparatuses and reaction conditions have been proposed. Examples known as a method using an apparatus are techniques using a specified mixing apparatus as a mixing apparatus for a surface-crosslinking agent (Patent Documents 26 to 29) and techniques using a heating apparatus for causing reaction of a water absorbent resin and a surface-crosslinking agent (Patent Documents 30, 31) and the like.
There is also a technique for controlling an increase in heating temperature for causing reaction of a water absorbent resin and a surface-crosslinking agent (Patent Document 32) in improvement of the reaction condition aspect. In a heating step, techniques known are a technique of carrying out surface-crosslinking twice (Patent Document 33), a technique of controlling particle size by drying a water absorbent resin, thereafter carrying out a second heat drying step, and further carrying out surface-crosslinking (Patent Document 34), a technique of defining oxygen partial pressure (Patent Document 35), techniques of defining the spraying conditions and dew points (Patent Documents 37, 38), techniques of defining the mixing conditions of treatment liquids (Patent Documents 39, 40), and a technique paying attention to a cooling step (Patent Document 41).